sys@LUOCS12C> !oerr ora 6509665096, 00000, "invalid common user or role name"// *Cause: An attempt was made to create a common user or role with a name// that wass not valid for common users or roles. In addition to // the usual rules for user and role names, common user and role // names must start with C## or c## and consist only of ASCII // characters.// *Action: Specify a valid common user or role name.//根据错误提示了解，在CDB中用户得以C##开头，如下：sys@LUOCS12C> create user c##luocs identified by oracle default tablespace luocs;

关于CONTAINER容器这个概念，官方介绍：The data dictionary in each container in a CDB is separate, and the current container is the container whose data dictionary is used for name resolution and for privilege authorization. The current container can be the root or a PDB. Each session has exactly one current container at any point in time, but it is possible for a session to switch from one container to another.Each container has a unique ID and name in a CDB. You can use the CON_ID and CON_NAME parameters in the USERENV namespace to determine the current container ID and name with the SYS_CONTEXT function.

下面我们看看12c新特性部分 Increased Size Limit for VARCHAR2, NVARCHAR2, and RAW Data Types官方如下介绍：The maximum size of the VARCHAR2, NVARCHAR2, and RAW data types has been increased from 4,000 to 32,767 bytes.Increasing the allotted size for these data types allows users to store more information in character data types before switching to large objects (LOBs). This is especially useful for brief textual data types and the capabilities to build indexes on these types of columns.

该参数设置为EXTENDED，上述类型才能支持到32767字节。但该参数并不是简单使用alter去修改就能解决的事情，如下：sys@LUOCS12C> alter system set MAX_STRING_SIZE=EXTENDED;alter system set MAX_STRING_SIZE=EXTENDED*ERROR at line 1:ORA-02097: parameter cannot be modified because specified value is invalidORA-14694: database must in UPGRADE mode to begin MAX_STRING_SIZE migration

下面再看一下另一个新特性：Native SQL Support for Query Row Limits and Row Offsets官方介绍如下：The FETCH FIRST and OFFSET clauses provides native SQL language support to limit the number of rows returned and to specify a starting row for the return set.Many queries need to limit the number of rows returned or offset the starting row of the results. For example, top-N queries sort their result set and then return only the first n rows. FETCH FIRST and OFFSET simplify syntax and comply with the ANSI SQL standard.